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{{expert-subject|date=November 2010}}
 
In [[statistical inference]], specifically [[predictive inference]], a '''prediction interval''' is an estimate of an interval in which future observations will fall, with a certain probability, given what has already been observed. Prediction intervals are often used in [[regression analysis]].
 
Prediction intervals are used in both [[frequentist statistics]] and [[Bayesian statistics]]: a prediction interval bears the same relationship to a future observation that a frequentist [[confidence interval]] or Bayesian [[credible interval]] bears to an unobservable population parameter: prediction intervals predict the distribution of individual future points, whereas confidence intervals and credible intervals of parameters predict the distribution of estimates of the true population mean or other quantity of interest that cannot be observed. Prediction intervals are also present in forecasts. It is difficult to estimate the prediction intervals of forecasts that have contrary series.<ref>{{cite journal |url=http://www.forecastingprinciples.com/paperpdf/Idenification%20of%20Asymmetric%20Prediction%20Intervals.pdf |title=Identification of Asymmetric Prediction Intervals through Causal Forces |last1=Armstrong |first1=J. Scott |last2=Collopy |first2=Fred |journal=Journal of Forecasting |volume=20 |pages=273–283 |year=2001}}</ref>
 
== Introduction ==
For example, if one makes the [[parametric statistics|parametric assumption]] that the underlying distribution is a [[normal distribution]], and has a sample set {''X''<sub>1</sub>,&nbsp;...,&nbsp;''X''<sub>''n''</sub>}, then confidence intervals and credible intervals may be used to estimate the [[population mean]] ''μ'' and [[population standard deviation]] ''σ'' of the underlying population, while prediction intervals may be used to estimate the value of the next sample variable, ''X''<sub>''n''+1</sub>.
 
Alternatively, in [[#Bayesian statistics|Bayesian terms]], a prediction interval can be described as a credible interval for the variable itself, rather than for a parameter of the distribution thereof.
 
The concept of prediction intervals need not be restricted to inference just a single future sample value but can be extended to more complicated cases. For example, in the context of river flooding where analyses are often based on annual values of the largest flow within the year, there may be interest in making inferences about the largest flood likely to be experienced within the next 50 years.
 
Since prediction intervals are only concerned with past and future observations, rather than unobservable population parameters, they are advocated as a better method than confidence intervals by some statisticians, such as [[Seymour Geisser]],{{Citation needed|date=August 2009}} following the focus on observables by [[Bruno de Finetti]].{{Citation needed|date=August 2009}}
 
== Normal distribution ==
Given a sample from a [[normal distribution]], whose parameters are unknown, it is possible to give prediction intervals in the frequentist sense, i.e., an interval [''a'',&nbsp;''b''] based on statistics of the sample such that on repeated experiments, ''X''<sub>''n''+1</sub> falls in the interval the desired percentage of the time; one may call these "predictive [[confidence interval]]s".<ref>{{Harv|Geisser|1993|loc=[http://books.google.com/books?id=wfdlBZ_iwZoC&printsec=frontcover#PPA6,M1 2.1.1 Confidence regions for future realizations: Parametric case, p. 6–]}}</ref>
 
A general technique of frequentist prediction intervals is to find and compute a [[pivotal quantity]] of the observables ''X''<sub>1</sub>,&nbsp;...,&nbsp;''X''<sub>''n''</sub>,&nbsp;''X''<sub>''n''+1</sub> – meaning a function of observables and parameters whose probability distribution does not depend on the parameters – that can be inverted to give a probability of the future observation ''X''<sub>''n''+1</sub> falling in some interval computed in terms of the observed values so far, <math>X_1,\dots,X_n.</math> Such a pivotal quantity, depending only on observables, is called an [[ancillary statistic]].<ref>{{Harv|Geisser|1993|loc=[http://books.google.com/books?id=wfdlBZ_iwZoC&printsec=frontcover#PPA7,M1 p. 7]}}</ref> The usual method of constructing pivotal quantities is to take the difference of two variables that depend on location, so that location cancels out, and then take the ratio of two variables that depend on scale, so that scale cancels out.
The most familiar pivotal quantity is the [[Student's t-statistic]], which can be derived by this method and is used in the sequel.
 
=== Known mean, known variance ===
A prediction interval [''l'',''u''] for a future observation ''X'' in a normal distribution ''N''(µ,σ<sup>2</sup>) with known [[mean]] and [[variance]] may easily be calculated from
 
:<math>\gamma=P(l<X<u)=P\left(\frac{l-\mu}{\sigma}<\frac{X-\mu}{\sigma}<\frac{u-\mu}{\sigma}\right)=P\left(\frac{l-\mu}{\sigma}<Z<\frac{u-\mu}{\sigma}\right),</math>
 
where <math>Z=\frac{X-\mu}{\sigma}</math>, the [[standard score]] of ''X'', is standard normal distributed.
 
Hence
:<math>\frac{l-\mu}{\sigma}=-z,\frac{u-\mu}{\sigma}=z,</math>
or
:<math>l=\mu-z\sigma,u=\mu+z\sigma,</math>
 
with ''z'' the [[quantile]] in the standard normal distribution for which:
 
:<math>\gamma=P(-z<Z<z).</math>
or equivalently;
 
:<math>\tfrac 12(1-\gamma)=P(Z>z).</math>
 
{|class="wikitable" align="left" style="margin-right:1em;"
! Prediction<br> interval !! z
|-
| 50% || 0.67<ref name=MedicalStatisticsA2>Page 472 (Table A2) in: {{cite book |last=Sterne |first=Jonathan |last2=Kirkwood |first2=Betty R. |title=Essential medical statistics |publisher=Blackwell Science |location=Oxford |year=2003 |page=472 |isbn=0-86542-871-9}}</ref>
|-
| 90% || 1.64<ref name=MedicalStatisticsA2/>
|-
| 95% || 1.96<ref name=MedicalStatisticsA2/>
|-
| 99% || 2.58<ref name=MedicalStatisticsA2/>
|}
[[File:Standard score and prediction interval.svg|thumb|250px|right|Prediction interval (on the [[y-axis]]) given from z (the quantile of the [[standard score]], on the [[x-axis]]). The y-axis is logarithmically compressed (but the values on it are not modified).]]
 
The prediction interval is conventionally written  as:
:<math>\left[\mu- z\sigma,\  \mu + z\sigma \right]. </math>
 
For example, to calculate the 95% prediction interval for a normal distribution with a mean (''µ'') of 5 and a standard deviation (''σ'') of 1, then z is approximately 2. Therefore, the lower limit of the prediction interval is approximately 5&nbsp;‒&nbsp;(2&middot;1) = 3, and the upper limit is approximately 5&nbsp;+&nbsp;(2&middot;1) = 7, thus giving a prediction interval of approximately 3 to 7.
 
[[File:Cumulative distribution function for normal distribution, mean 0 and sd 1.svg|270px|thumb|right|Diagram showing the [[cumulative distribution function]] for the normal distribution with mean (''µ'') 0 and variance (''σ''<sup>2</sup>)&nbsp;1. In addition to the [[quantile function]], the prediction interval for any standard score can be calculated by (1&nbsp;&minus;&nbsp;(1&nbsp;&minus;&nbsp;<span style="font-size:100%;">Φ</span><sub>''µ'',''σ''<sup>2</sup></sub>(standard score))&middot;2). For example, a standard score of ''x''&nbsp;=&nbsp;1.96 gives <span style="font-size:100%;">Φ</span><sub>''µ'',''σ''<sup>2</sup></sub>(1.96)&nbsp;=&nbsp;0.9750 corresponding to a prediction interval of (1&nbsp;&minus;&nbsp;(1&nbsp;&minus;&nbsp;0.9750)&middot;2) =&nbsp;0.9500&nbsp;=&nbsp;95%.]]
 
=== Estimation of parameters ===
For a distribution with unknown parameters, a direct approach to prediction is to estimate the parameters and then use the associated quantile function – for example, one could use the sample mean <math>\overline{X}</math> as estimate for ''μ'' and the [[sample variance]] ''s''<sup>2</sup> as an estimate for ''σ''<sup>2</sup>. Note that there are two natural choices for ''s''<sup>2</sup> here – dividing by <math>(n-1)</math> yields an unbiased estimate, while dividing by ''n'' yields the [[maximum likelihood estimator]], and either might be used. One then uses the quantile function with these estimated parameters <math>\Phi^{-1}_{\overline{X},s^2}</math> to give a prediction interval.
 
This approach is usable, but the resulting interval will not have the repeated sampling interpretation<ref>{{Harv|Geisser|1993|loc=[http://books.google.com/books?id=wfdlBZ_iwZoC&printsec=frontcover#PPA8,M1 p. 8–9]}}</ref> – it is not a predictive confidence interval.
 
For the sequel, use the sample mean:
:<math>\overline{X} = \overline{X}_n=(X_1+\cdots+X_n)/n</math>
and the (unbiased) sample variance:
:<math>s^2 = s_n^2={1 \over n-1}\sum_{i=1}^n (X_i-\overline{X}_n)^2.</math>
 
==== Unknown mean, known variance ====
Given<ref>{{Harv|Geisser|1993|loc=[http://books.google.com/books?id=wfdlBZ_iwZoC&printsec=frontcover#PPA7,M1 p. 7–]}}</ref> a normal distribution with unknown mean ''μ'' but known variance 1, the sample mean <math>\overline{X}</math> of the observations <math>X_1,\dots,X_n</math> has distribution <math>N(\mu,1/n),</math> while the future observation <math>X_{n+1}</math> has distribution <math>N(\mu,1).</math> Taking the difference of these cancels the ''μ'' and yields a normal distribution of variance <math>1+(1/n),</math> thus
:<math>\frac{X_{n+1}-\overline{X}}{\sqrt{1+(1/n)}} \sim N(0,1).</math>
Solving for <math>X_{n+1}</math> gives the prediction distribution <math>N(\overline{X},1+(1/n)),</math> from which one can compute intervals as before. This is a predictive confidence interval in the sense that if one uses a quantile range of 100''p''%, then on repeated applications of this computation, the future observation <math>X_{n+1}</math> will fall in the predicted interval 100''p''% of the time.
 
Notice that this prediction distribution is more conservative than using the estimated mean <math>\overline{X}</math> and known variance 1, as this uses variance <math>1+(1/n)</math>, hence yields wider intervals. This is necessary for the desired confidence interval property to hold.
 
==== Known mean, unknown variance ====
Conversely, given a normal distribution with known mean 0 but unknown variance <math>\sigma^2</math>,
the sample variance <math>s^2</math> of the observations <math>X_1,\dots,X_n</math> has, up to scale, a [[chi-squared distribution|<math>\scriptstyle\chi_{n-1}^2</math> distribution]]; more precisely:
:<math>\frac{(n-1)s_n^2}{\sigma^2} \sim \chi_{n-1}^2.</math>
while the future observation <math>X_{n+1}</math> has distribution <math>N(0,\sigma^2).</math>
Taking the ratio of the future observation and the sample standard deviation cancels the ''σ,'' yielding a [[Student's t-distribution]] with ''n''–1 [[degrees of freedom (statistics)|degrees of freedom]]:
:<math>\frac{X_{n+1}}{s} \sim T^{n-1}.</math>
Solving for <math>X_{n+1}</math> gives the prediction distribution <math>sT^{n-1},</math> from which one can compute intervals as before.
 
Notice that this prediction distribution is more conservative than using a normal distribution with the estimated standard deviation <math>s</math> and known mean 0, as it uses the t-distribution instead of the normal distribution, hence yields wider intervals. This is necessary for the desired confidence interval property to hold.
 
==== Unknown mean, unknown variance ====
Combining the above for a normal distribution <math>N(\mu,\sigma^2)</math> with both ''μ'' and ''σ''<sup>2</sup> unknown yields the following ancillary statistic:<ref>{{Harv|Geisser|1993|loc=[http://books.google.com/books?id=wfdlBZ_iwZoC&printsec=frontcover#PPA9,M1 Example 2.2, p. 9–10]}}</ref>
:<math>\frac{X_{n+1}-\overline{X}_n}{s_n\sqrt{1+1/n}} \sim T^{n-1}.</math>
This simple combination is possible because the sample mean and sample variance of the normal distribution are independent statistics; this is only true for the normal distribution, and in fact characterizes the normal distribution.
 
Solving for <math>X_{n+1}</math> yields the prediction distribution
:<math>\overline{X}_n + s_n\sqrt{1+1/n} \cdot T^{n-1}.</math>
 
The probability of <math>X_{n+1}</math> falling in a given interval is then:
:<math>\Pr\left(\overline{X}_n-T_a s_n\sqrt{1+(1/n)}\leq X_{n+1}  \leq\overline{X}_n+T_a s_n\sqrt{1+(1/n)}\,\right)=p</math>
 
where ''T<sub>a</sub>'' is the 100((1&nbsp;+&nbsp;''p'')/2)<sup>th</sup> [[percentile]] of [[Student's t-distribution]] with ''n'' &minus; 1 degrees of freedom.  Therefore the numbers
 
:<math>\overline{X}_n\pm T_a {s}_n\sqrt{1+(1/n)}</math>
 
are the endpoints of a 100''p''% prediction interval for <math>X_{n+1}</math>.
 
== Non-parametric methods ==
 
One can compute prediction intervals without any assumptions on the population; formally, this is a [[non-parametric statistics|non-parametric]] method.<ref>[http://www.oswego.edu/~srp/stats/pi.htm Prediction Intervals], Statistics @ SUNY Oswego</ref>
 
Suppose one randomly draws a sample of two observations ''X''<sub>1</sub> and ''X''<sub>2</sub> from a population in which values are assumed to have a [[continuous probability distribution]]
 
:What is the probability that ''X''<sub>2</sub> > ''X''<sub>1</sub>?
 
The answer is exactly 50%, ''regardless'' of the underlying population – the probability of picking 3 and then 7 is the same as picking 7 and then 3, regardless of the particular probability of picking 3 or&nbsp;7. Thus, if one picks a single sample ''X''<sub>1</sub>, then 50% of the time the next sample will be greater, which yields (''X''<sub>1</sub>,&nbsp;+∞) as a 50% prediction interval for ''X''<sub>2</sub>.  Similarly, 50% of the time it will be smaller, which yields another 50% prediction interval for ''X''<sub>2</sub>, namely (&minus;∞,&nbsp;''X''<sub>1</sub>). Note that the assumption of a continuous distribution avoids the possibililty that values might be exactly equal; this would complicate matters.
 
Similarly, if one has a sample {''X''<sub>1</sub>,&nbsp;...,&nbsp;''X''<sub>''n''</sub>} then the probability that the next observation ''X''<sub>''n''+1</sub> will be the largest is 1/(''n''&nbsp;+&nbsp;1), since all observations have equal probability of being the maximum. In the same way, the probability that ''X''<sub>''n''+1</sub> will be the smallest is 1/(''n''&nbsp;+&nbsp;1). The other (''n''&nbsp;&minus;&nbsp;1)/(''n''&nbsp;+&nbsp;1) of the time, ''X''<sub>''n''+1</sub> falls between the [[sample maximum]] and [[sample minimum]] of the sample {''X''<sub>1</sub>,&nbsp;...,&nbsp;''X''<sub>''n''</sub>}. Thus, denoting the sample maximum and minimum by ''M'' and ''m,'' this yields an (''n''&nbsp;&minus;&nbsp;1)/(''n''&nbsp;+&nbsp;1) prediction interval of [''m'',&nbsp;''M''].
 
For example, if ''n''&nbsp;=&nbsp;19, then [''m'',&nbsp;''M''] gives an 18/20 =&nbsp;90% prediction interval – 90% of the time, the 20th observation falls between the smallest and largest observation seen heretofore. Likewise, ''n''&nbsp;=&nbsp;39 gives a 95% prediction interval, and ''n''&nbsp;=&nbsp;199 gives a 99% prediction interval.
 
One can visualize this by drawing the ''n'' samples on a line, which divides the line into ''n''&nbsp;+&nbsp;1 sections (''n''&nbsp;&minus;&nbsp;1 segments between samples, and 2 intervals going to infinity at both ends), and noting that ''X''<sub>''n''+1</sub> has an equal chance of landing in any of these ''n''&nbsp;+&nbsp;1 sections. Thus one can also pick any ''k'' of these sections and give a ''k''/(''n''&nbsp;+&nbsp;1) prediction interval (or set, if the sections are not consecutive). For instance, if ''n''&nbsp;=&nbsp;2, then the probability that ''X''<sub>3</sub> will land between the existing 2 observations is&nbsp;1/3.
 
Notice that while this gives the probability that a future observation will fall in a range, it does not give any estimate as to where in a segment it will fall – notably, if it falls outside the range of observed values, it may be far outside the range. See [[extreme value theory]] for further discussion. Formally, this applies not just to sampling from a population, but to any [[exchangeable sequence]] of random variables, not necessarily independent or [[identically distributed]].
 
== Contrast with other intervals ==
{{Main|Interval estimation}}
 
=== Contrast with confidence intervals ===
{{main|Confidence interval}}
Note that in the formula for the predictive confidence interval ''no mention'' is made of the unobservable parameters ''μ'' and ''σ'' of population mean and standard deviation – the observed ''sample'' statistics <math>\overline{X}_n</math> and <math>S_n</math> of sample mean and standard deviation are used, and what is estimated is the outcome of ''future'' samples.
 
Rather than using sample statistics as estimators of population parameters and applying confidence intervals to these estimates, one considers "the next sample" <math>X_{n+1}</math> as ''itself'' a statistic, and computes its [[sampling distribution]].
 
In parameter confidence intervals, one estimates population parameters; if one wishes to interpret this as prediction of the next sample, one models "the next sample" as a draw from this estimated population, using the (estimated) ''population'' distribution. By contrast, in predictive confidence intervals, one uses the ''sampling'' distribution of (a statistic of) ''n'' or ''n''+1 samples from such a population, and the population distribution is not directly used, though the assumption about its form (though not the values of its parameters) is used in computing the sampling distribution.
 
=== Contrast with tolerance intervals ===
{{main|Tolerance interval}}
{{Empty section|date=February 2013}}
 
==Applications==
Prediction intervals are commonly used as definitions of [[reference range]]s, such as [[reference ranges for blood tests]] to give an idea of whether a [[blood test]] is normal or not. For this purpose, the most commonly used prediction interval is the 95% prediction interval, and a reference range based on it can be called a ''standard reference range''.
 
=== Regression analysis ===
 
A common application of prediction intervals is to [[regression analysis]].
 
Suppose the data is being modeled by a straight line regression:
:<math>y_i=\alpha+\beta x_i +\epsilon_i\,</math>
where <math>y_i</math> is the [[response variable]], <math>x_i</math> is the [[explanatory variable]], ''ε<sub>i</sub>'' is a random error term, and <math>\alpha</math> and <math>\beta</math> are parameters.
 
Given estimates <math>\hat \alpha</math> and <math>\hat \beta</math> for the parameters, such as from a [[simple linear regression]], the predicted response value ''y<sub>d</sub>'' for a given explanatory value ''x<sub>d</sub>'' is
:<math>\hat{y}_d=\hat\alpha+\hat\beta x_d ,</math>
(the point on the regression line), while the actual response would be
:<math>y_d=\alpha+\beta x_d +\epsilon_d.  \,</math>
The [[point estimate]] <math>\hat{y}_d</math> is called the [[mean response]], and is an estimate of the [[expected value]] of ''y<sub>d</sub>,'' <math>E(y|x_d).</math>
 
A prediction interval instead gives an interval in which one expects ''y<sub>d</sub>'' to fall; this is not necessary if the actual parameters ''α'' and ''β'' are known (together with the error term ''ε<sub>i</sub>''), but if one is estimating from a [[Sampling (statistics)|sample]], then one may use the [[standard error]] of the estimates for the intercept and slope (<math>\hat\alpha</math> and <math>\hat\beta</math>), as well as their correlation, to compute a prediction interval.
 
In regression, Faraway (2002, p.39) makes a distinction between intervals for predictions of the mean response vs. for predictions of observed response -- affecting essentially the inclusion or not of the unity term within the square root in the expansion factors above; see <ref name=Faraway>J. J. Faraway, "Practical Regression and Anova using R", [http://cran.r-project.org/doc/contrib/Faraway-PRA.pdf]</ref> for details.
 
== Bayesian statistics ==
[[Seymour Geisser]], a proponent of predictive inference, gives predictive applications of [[Bayesian statistics]].<ref>{{Harv|Geisser|1993}}</ref>
 
In Bayesian statistics, one can compute (Bayesian) prediction intervals from the [[posterior probability]] of the random variable, as a [[credible interval]]. In theoretical work, credible intervals are not often calculated for the prediction of future events, but for inference of parameters – i.e., credible intervals of a parameter, not for the outcomes of the variable itself. However, particularly where applications are concerned with possible extreme values of yet to be observed cases, credible intervals for such values can be of practical importance.
 
==See also==
{{colbegin}}
*[[Confidence interval]]
*[[Extrapolation]]
*[[Posterior probability]]
*[[Prediction]]
*[[Confidence and prediction bands|Prediction band]]
*[[Regression analysis]]
*[[Seymour Geisser]]
*[[Trend estimation]]
{{colend}}
 
==Further reading==
*"Confidence intervals for predictions", chap. 3.5 in <ref name=Faraway/>
*{{cite journal | doi = 10.2307/1391361 | last1 = Chatfield | first1 = C. | year = 1993 | title = Calculating Interval Forecasts | url = | journal = Journal of Business and Economic Statistics | volume = 11 | issue = 2| pages = 121–135 | ref = harv }}
* {{Cite book
|url=http://books.google.com/?id=wfdlBZ_iwZoC
|title=Predictive Inference: An Introduction
|first=Seymour
|last=Geisser
|authorlink=Seymour Geisser
|publisher=[[CRC Press]]
|isbn=0-412-03471-9
|pages=280
|year=1993
|ref=harv
|postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}
}}
*{{cite journal | last1 = Meade | first1 = N. | last2 = Islam | first2 = T. | year = 1995 | title = Prediction Intervals for Growth Curve Forecasts | doi =10.1002/for.3980140502| journal = Journal of Forecasting | volume = 14 | issue = 5| pages = 413–430 | ref = harv }}
*{{cite journal | doi = 10.1093/biomet/92.3.529 | last1 = Lawless | first1 = J.F. | last2 = Fredette | first2 = M. | year = 2005 | title = Frequentist prediction intervals and predictive distributions | url = | journal = Biometrika | volume = 92 | issue = 3| pages = 529–542 | ref = harv }}
* ISO 16269-8 Standard Interpretation of Data, Part 8, Determination of Prediction Intervals
 
==References==
{{reflist}}
 
{{DEFAULTSORT:Prediction Interval}}
[[Category:Statistical terminology]]
[[Category:Statistical inference]]
[[Category:Regression analysis]]
[[Category:Statistical intervals]]

Revision as of 02:33, 12 March 2013

Template:Expert-subject

In statistical inference, specifically predictive inference, a prediction interval is an estimate of an interval in which future observations will fall, with a certain probability, given what has already been observed. Prediction intervals are often used in regression analysis.

Prediction intervals are used in both frequentist statistics and Bayesian statistics: a prediction interval bears the same relationship to a future observation that a frequentist confidence interval or Bayesian credible interval bears to an unobservable population parameter: prediction intervals predict the distribution of individual future points, whereas confidence intervals and credible intervals of parameters predict the distribution of estimates of the true population mean or other quantity of interest that cannot be observed. Prediction intervals are also present in forecasts. It is difficult to estimate the prediction intervals of forecasts that have contrary series.[1]

Introduction

For example, if one makes the parametric assumption that the underlying distribution is a normal distribution, and has a sample set {X1, ..., Xn}, then confidence intervals and credible intervals may be used to estimate the population mean μ and population standard deviation σ of the underlying population, while prediction intervals may be used to estimate the value of the next sample variable, Xn+1.

Alternatively, in Bayesian terms, a prediction interval can be described as a credible interval for the variable itself, rather than for a parameter of the distribution thereof.

The concept of prediction intervals need not be restricted to inference just a single future sample value but can be extended to more complicated cases. For example, in the context of river flooding where analyses are often based on annual values of the largest flow within the year, there may be interest in making inferences about the largest flood likely to be experienced within the next 50 years.

Since prediction intervals are only concerned with past and future observations, rather than unobservable population parameters, they are advocated as a better method than confidence intervals by some statisticians, such as Seymour Geisser,Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park. following the focus on observables by Bruno de Finetti.Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park.

Normal distribution

Given a sample from a normal distribution, whose parameters are unknown, it is possible to give prediction intervals in the frequentist sense, i.e., an interval [ab] based on statistics of the sample such that on repeated experiments, Xn+1 falls in the interval the desired percentage of the time; one may call these "predictive confidence intervals".[2]

A general technique of frequentist prediction intervals is to find and compute a pivotal quantity of the observables X1, ..., XnXn+1 – meaning a function of observables and parameters whose probability distribution does not depend on the parameters – that can be inverted to give a probability of the future observation Xn+1 falling in some interval computed in terms of the observed values so far, Such a pivotal quantity, depending only on observables, is called an ancillary statistic.[3] The usual method of constructing pivotal quantities is to take the difference of two variables that depend on location, so that location cancels out, and then take the ratio of two variables that depend on scale, so that scale cancels out. The most familiar pivotal quantity is the Student's t-statistic, which can be derived by this method and is used in the sequel.

Known mean, known variance

A prediction interval [l,u] for a future observation X in a normal distribution N(µ,σ2) with known mean and variance may easily be calculated from

where , the standard score of X, is standard normal distributed.

Hence

or

with z the quantile in the standard normal distribution for which:

or equivalently;

Prediction
interval
z
50% 0.67[4]
90% 1.64[4]
95% 1.96[4]
99% 2.58[4]
Prediction interval (on the y-axis) given from z (the quantile of the standard score, on the x-axis). The y-axis is logarithmically compressed (but the values on it are not modified).

The prediction interval is conventionally written as:

For example, to calculate the 95% prediction interval for a normal distribution with a mean (µ) of 5 and a standard deviation (σ) of 1, then z is approximately 2. Therefore, the lower limit of the prediction interval is approximately 5 ‒ (2·1) = 3, and the upper limit is approximately 5 + (2·1) = 7, thus giving a prediction interval of approximately 3 to 7.

Diagram showing the cumulative distribution function for the normal distribution with mean (µ) 0 and variance (σ2) 1. In addition to the quantile function, the prediction interval for any standard score can be calculated by (1 − (1 − Φµ,σ2(standard score))·2). For example, a standard score of x = 1.96 gives Φµ,σ2(1.96) = 0.9750 corresponding to a prediction interval of (1 − (1 − 0.9750)·2) = 0.9500 = 95%.

Estimation of parameters

For a distribution with unknown parameters, a direct approach to prediction is to estimate the parameters and then use the associated quantile function – for example, one could use the sample mean as estimate for μ and the sample variance s2 as an estimate for σ2. Note that there are two natural choices for s2 here – dividing by yields an unbiased estimate, while dividing by n yields the maximum likelihood estimator, and either might be used. One then uses the quantile function with these estimated parameters to give a prediction interval.

This approach is usable, but the resulting interval will not have the repeated sampling interpretation[5] – it is not a predictive confidence interval.

For the sequel, use the sample mean:

and the (unbiased) sample variance:

Unknown mean, known variance

Given[6] a normal distribution with unknown mean μ but known variance 1, the sample mean of the observations has distribution while the future observation has distribution Taking the difference of these cancels the μ and yields a normal distribution of variance thus

Solving for gives the prediction distribution from which one can compute intervals as before. This is a predictive confidence interval in the sense that if one uses a quantile range of 100p%, then on repeated applications of this computation, the future observation will fall in the predicted interval 100p% of the time.

Notice that this prediction distribution is more conservative than using the estimated mean and known variance 1, as this uses variance , hence yields wider intervals. This is necessary for the desired confidence interval property to hold.

Known mean, unknown variance

Conversely, given a normal distribution with known mean 0 but unknown variance , the sample variance of the observations has, up to scale, a distribution; more precisely:

while the future observation has distribution Taking the ratio of the future observation and the sample standard deviation cancels the σ, yielding a Student's t-distribution with n–1 degrees of freedom:

Solving for gives the prediction distribution from which one can compute intervals as before.

Notice that this prediction distribution is more conservative than using a normal distribution with the estimated standard deviation and known mean 0, as it uses the t-distribution instead of the normal distribution, hence yields wider intervals. This is necessary for the desired confidence interval property to hold.

Unknown mean, unknown variance

Combining the above for a normal distribution with both μ and σ2 unknown yields the following ancillary statistic:[7]

This simple combination is possible because the sample mean and sample variance of the normal distribution are independent statistics; this is only true for the normal distribution, and in fact characterizes the normal distribution.

Solving for yields the prediction distribution

The probability of falling in a given interval is then:

where Ta is the 100((1 + p)/2)th percentile of Student's t-distribution with n − 1 degrees of freedom. Therefore the numbers

are the endpoints of a 100p% prediction interval for .

Non-parametric methods

One can compute prediction intervals without any assumptions on the population; formally, this is a non-parametric method.[8]

Suppose one randomly draws a sample of two observations X1 and X2 from a population in which values are assumed to have a continuous probability distribution

What is the probability that X2 > X1?

The answer is exactly 50%, regardless of the underlying population – the probability of picking 3 and then 7 is the same as picking 7 and then 3, regardless of the particular probability of picking 3 or 7. Thus, if one picks a single sample X1, then 50% of the time the next sample will be greater, which yields (X1, +∞) as a 50% prediction interval for X2. Similarly, 50% of the time it will be smaller, which yields another 50% prediction interval for X2, namely (−∞, X1). Note that the assumption of a continuous distribution avoids the possibililty that values might be exactly equal; this would complicate matters.

Similarly, if one has a sample {X1, ..., Xn} then the probability that the next observation Xn+1 will be the largest is 1/(n + 1), since all observations have equal probability of being the maximum. In the same way, the probability that Xn+1 will be the smallest is 1/(n + 1). The other (n − 1)/(n + 1) of the time, Xn+1 falls between the sample maximum and sample minimum of the sample {X1, ..., Xn}. Thus, denoting the sample maximum and minimum by M and m, this yields an (n − 1)/(n + 1) prediction interval of [mM].

For example, if n = 19, then [mM] gives an 18/20 = 90% prediction interval – 90% of the time, the 20th observation falls between the smallest and largest observation seen heretofore. Likewise, n = 39 gives a 95% prediction interval, and n = 199 gives a 99% prediction interval.

One can visualize this by drawing the n samples on a line, which divides the line into n + 1 sections (n − 1 segments between samples, and 2 intervals going to infinity at both ends), and noting that Xn+1 has an equal chance of landing in any of these n + 1 sections. Thus one can also pick any k of these sections and give a k/(n + 1) prediction interval (or set, if the sections are not consecutive). For instance, if n = 2, then the probability that X3 will land between the existing 2 observations is 1/3.

Notice that while this gives the probability that a future observation will fall in a range, it does not give any estimate as to where in a segment it will fall – notably, if it falls outside the range of observed values, it may be far outside the range. See extreme value theory for further discussion. Formally, this applies not just to sampling from a population, but to any exchangeable sequence of random variables, not necessarily independent or identically distributed.

Contrast with other intervals

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church.

Contrast with confidence intervals

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Note that in the formula for the predictive confidence interval no mention is made of the unobservable parameters μ and σ of population mean and standard deviation – the observed sample statistics and of sample mean and standard deviation are used, and what is estimated is the outcome of future samples.

Rather than using sample statistics as estimators of population parameters and applying confidence intervals to these estimates, one considers "the next sample" as itself a statistic, and computes its sampling distribution.

In parameter confidence intervals, one estimates population parameters; if one wishes to interpret this as prediction of the next sample, one models "the next sample" as a draw from this estimated population, using the (estimated) population distribution. By contrast, in predictive confidence intervals, one uses the sampling distribution of (a statistic of) n or n+1 samples from such a population, and the population distribution is not directly used, though the assumption about its form (though not the values of its parameters) is used in computing the sampling distribution.

Contrast with tolerance intervals

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Template:Empty section

Applications

Prediction intervals are commonly used as definitions of reference ranges, such as reference ranges for blood tests to give an idea of whether a blood test is normal or not. For this purpose, the most commonly used prediction interval is the 95% prediction interval, and a reference range based on it can be called a standard reference range.

Regression analysis

A common application of prediction intervals is to regression analysis.

Suppose the data is being modeled by a straight line regression:

where is the response variable, is the explanatory variable, εi is a random error term, and and are parameters.

Given estimates and for the parameters, such as from a simple linear regression, the predicted response value yd for a given explanatory value xd is

(the point on the regression line), while the actual response would be

The point estimate is called the mean response, and is an estimate of the expected value of yd,

A prediction interval instead gives an interval in which one expects yd to fall; this is not necessary if the actual parameters α and β are known (together with the error term εi), but if one is estimating from a sample, then one may use the standard error of the estimates for the intercept and slope ( and ), as well as their correlation, to compute a prediction interval.

In regression, Faraway (2002, p.39) makes a distinction between intervals for predictions of the mean response vs. for predictions of observed response -- affecting essentially the inclusion or not of the unity term within the square root in the expansion factors above; see [9] for details.

Bayesian statistics

Seymour Geisser, a proponent of predictive inference, gives predictive applications of Bayesian statistics.[10]

In Bayesian statistics, one can compute (Bayesian) prediction intervals from the posterior probability of the random variable, as a credible interval. In theoretical work, credible intervals are not often calculated for the prediction of future events, but for inference of parameters – i.e., credible intervals of a parameter, not for the outcomes of the variable itself. However, particularly where applications are concerned with possible extreme values of yet to be observed cases, credible intervals for such values can be of practical importance.

See also

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Further reading

  • "Confidence intervals for predictions", chap. 3.5 in [9]
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    A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

    There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  • One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

    In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

    Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

    Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

    A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

    There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
  • One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

    In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

    Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

    Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

    A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

    There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
  • ISO 16269-8 Standard Interpretation of Data, Part 8, Determination of Prediction Intervals

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

  1. One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

    In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

    Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

    Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

    A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

    There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
  2. Template:Harv
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  4. 4.0 4.1 4.2 4.3 Page 472 (Table A2) in: 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  5. Template:Harv
  6. Template:Harv
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  8. Prediction Intervals, Statistics @ SUNY Oswego
  9. 9.0 9.1 J. J. Faraway, "Practical Regression and Anova using R", [1]
  10. Template:Harv